The present invention generally relates to sailing vessels and more particularly to means for automatically adjusting the tension of the luff, foot, and leech of a mast-rigged sail while under way.
The camber of a sail seriously affects the speed with which it will propel a vessel. The optimum camber depends on many factors including the size, shape, and potential speed of the vessel; sea conditions, for water-borne vessels; the wind velocity; and the angle to the wind at which the vessel is sailing. The camber is partly determined during manufacture of the sail, but is also substantially affected by the tension on its luff, foot, and leech while in operation.
Current practice is to make adjustments to a mast-rigged sail manually, and separately for each of the three sides of the sail. The sail is held at the top of the mast with a halyard, which is not adjusted while under way.
A cunningham, a line secured to the tack of the sail and led downward, is commonly used to adjust the tension along the luff. Alternatively, the tack of the sail is secured to the front of the boom; the boom is attached to the mast on a vertical slide means; and a downhaul, a line secured to the front of the boom and led downward, is used to adjust the tension along the luff.
An outhaul, a line secured to the clew of the sail and led to about the distal end of the boom, is used to adjust the tension along the foot of the sail.
The tension along the leech of a sail is adjusted by several means. When the vessel is beating, the primary means usually is the sheetline to the boom. When the vessel is off the wind, the primary means usually is the vang. In addition, adjustments of the outhaul and headstay, and deflection of the mast, affect the leech tension.
The optimal camber for a sail is generally achieved by increasing the tensions along the luff, foot, and leech as the vessel heads closer into the wind, and reducing these tensions as the vessel heads farther away from the wind. In addition, as the wind velocity increases, the tensions should be increased; and as the wind drops, the tensions should be reduced. It is not uncommon in racing vessels for the crew to make 10 or 15 adjustments to the sail tensions each hour.
A few prior art vessels have automatic adjustment of the tension along the foot of a sail for changes in the heading of the vessel. The forward end of the boom is mounted on the deck substantially aft of the mast. Since the sail's vertical axis of pivot is along the transverse center of the aft edge of the mast, this makes the boom eccentric to the sail in the horizontal plane. As the boom is sheeted out, the distal end of the boom gradually moves closer to the aft edge of the mast, thus easing the tension along the foot of the sail. These deck-mounted booms increase the stresses on the deck, require vangs to be the expensive radiused type, and usually present serious inconveniences and hazards to the crew.
On modern sailing vessels the boom is almost always mounted to the aft edge of a mast some distance above the deck. The mounting means, a gooseneck, is a pivoting mechanism that includes a vertical axis so that the boom can rotate in a horizontal plane and a transverse axis so that the boom can rotate in a vertical plane. The vertical axis is as close to the mast as convenient for its construction. In vessels of ten to twenty feet in length, that is about three-quarters of an inch. In larger vessels it is correspondingly farther, but still relatively close to the mast. To set the vertical axis of a gooseneck substantially aft of the mast creates structural complications, aesthetic problems, and considerable torsional stress on the mast when the boom is sheeted out.
Prior art boom vangs often make small adjustments to the tension of the leech automatically in response to sheeting the boom in and out. For them not to, the lower end of the vang must be secured directly on a projection of the boom's vertical axis of pivot or on a semicircular fitting centered directly about the projection. If the vang is secured a half inch forward of the boom's vertical axis of pivot, as it may be for structural or other reasons, the vang will be eccentric to the boom, and in such a manner that as the boom is sheeted out the vang will reduce the tension on the leech. If, however, the vang is secured a half of an inch aft, there will be an eccentricity such that as the boom is sheeted out the vang will increase the tension on the leech, rather than reduce it. In addition, when the lower end of the vang is secured to a transversely disposed bale, the arc of the fitting often allows the vang to slide on the bale more laterally than forward and aft, and this results in some easing of the leech tension as the boom is sheeted out. The amount of automatic adjustment of the leech tension caused by these small geometric asymmetries is usually quite modest, and often unrecognized by sailors. Manual adjustment of the vang is almost universal in racing vessels.
Individual manual adjustments of the luff, foot, and leech tensions are often difficult to do as accurately as desired. They require the crew to move their weight temporarily to positions that are otherwise not preferred. They are usually done less frequently and less promptly than wished because of the crew's preoccupation with other matters. And when being done, they distract the crew's attention from other responsibilities. At best, they are a hassle for the crew; at worst, they impede optimal functioning of the vessel.